Toxoplasma gondii parasites enter human populations by avenues such as the food and water supply where they cause serious pathology in individuals whose immune system is underdeveloped, diseased, or suppressed. Once exposed, Toxoplasma infections are permanent due to features of the lifecycle that allow the parasites to cycle indefinitely and escape or avoid immune countermeasures. Thus, damage to the immune system at any time during life brings an immediate risk of toxoplasmosis. Repeated switching between the lytic tachyzoite cycle, which is responsible for pathogenesis, and the dormant bradyzoite stage leads to progressive tissue destruction that accumulates until overt symptoms are evident. In AIDS patients, damage to the brain results in severe central nervous system complications that are implicated in the deaths of these patients. Recently, studies from our and other laboratories have established a link between the mechanism controlling bradyzoite switching and those which regulate the rate of tachyzoite growth. Defining the molecular elements of this process, therefore, has significant implications to understanding the underlying mechanisms responsible for chronic toxoplasmosis. This project seeks to dissect the most important biosynthetic pathway of the tachyzoite cell cycle, namely DNA replication, and its intersection with control of the developmental cycle. Specifically, this proposal aims: (1) to study the relationship between cell cycle and parasite development by characterizing a novel G2 population that arises during differentiation, (2) we will investigate T. gondii cyclin and cyclin-dependent kinase in order to better understand the control of this parasite's unusual cell cycle, and (3) we will explore the intersection between development and the chromosome cycle through studies of key DNA synthetic factors. Given the unusual nature of the cell cycle of these parasites, the proteins and regulatory pathways we intend to study here, provide a new direction for drug development in these important human and animal pathogens.